Acoustic Guided waves, such as Lamb waves, are typically used to carry out ultrasonic nondestructive evaluation (NDE) of thin-wall structures such as pipes, shells, membranes, and plates. Guided waves are preferred to bulk waves because they can travel long distances, thereby making it possible to inspect wide areas with fewer measurements. Guided waves are generally analyzed by the well-known Rayleigh-Lamb wave dispersion relationship, expressed in terms of the thickness of the material and certain material constants, such as the modulus of elasticity, Poisson's ratio, or wave velocities. In determining dispersion equations, a set of curves can be obtained which relates phase velocities and frequencies. Such a set of curves is shown in FIG. 1, which is graph of the multiple dispersion curves corresponding to propagation modes for waves in an aluminum plate of a thickness 2h.
Guided waves are both multi-modal and dispersive in nature. They are dispersive, meaning that waves oscillating in different frequencies travel at different speeds. In other words, phase velocity is not a constant value but a function of frequency. This means that the wave motion depends on the characteristics of the excitation signal. As a result, a broadband signal such as a spike pulse traveling in a dispersive medium may significantly change its shape as it propagates in the medium. On the other hand, the shape of an extremely narrowband signal, such as a tone burst signal, is preserved as it propagates in the medium.
Since broadband pulses are often too complicated and difficult to analyze, a more conventional approach is to use narrowband signals whose carrier frequency is swept over the width of the frequency band of interest. The advantage to this approach is that the signal retains its shape as it propagates in the medium. It is thus easier to analyze data and visualize the propagating and reflecting waves directly in the time domain.
In addition to dispersion, the other characteristic that distinguishes guided waves from bulk ultrasonic waves is their multi-modality. For a given thickness and frequency, there may exist many different propagation modes which are basically grouped into two different fundamental families: symmetric (S) and anti-symmetric (A) mode, such as those shown in FIG. 1. The Rayleigh-Lamb relationship yields infinitely many harmonic solutions for each mode. But, for NDE, it is desirable to differentiate one particular mode of propagation from the other modes, resulting in fewer peaks in the waveforms acquired.
Each dispersion curve corresponds to a particular mode of propagation and, for any given frequency, there exists at least two modes of propagation. These signals in their untuned state are generally too complicated to analyze and therefore it is necessary to distinguish a particular mode of interest from the other co-existing modes. Two common systems for generating guided waves in a selected mode are angle wedge tuners and array transducers. These systems are described separately below.
The most common system for generating guided waves is an angle wedge tuner or oblique angle insonification system. In general, a variable or fixed angle wedge transducer is used for controlling the incident angle of the applied signal. The wedge may be placed directly on the specimen, or alternatively, the insonification and detection can be made without direct contact using immersion and air-coupled transducers.
The basic principle for wedge tuning is Snell's law: ##EQU1##
where .theta..sub.w is the angle of incidence for tuning a selected mode propagating at the phase velocity c.sub.p, and c.sub.w is the longitudinal wave velocity in the wedge which typically is 2,720 m/s. Accordingly, once the carrier frequency of the tone burst signal, the thickness of the medium under test and the longitudinal wave velocity in the wedge are known, the graph of FIG. 1 may be used to determine the required incident angle to tune the signal to the selected mode.
Problems associated with the angle wedge transducer include the difficulty of accurately setting the angle of incidence, since the variable wedge is manipulated manually. Accordingly, the sensitivity due to misalignment is uncertain and error levels may vary for different modes and frequencies. Another drawback results from the numerous interfaces that the signal must traverse in the wedge assembly. Typically, a variable angle wedge transducer includes two parts, a main wedge and block rotating around the wedge. Since the transducer is mounted on the block, three interfaces exist in the transducer-wedge assembly: one between the transducer and the rotating block; one between the rotating block and the main wedge; and one between the wedge and the medium under test. These interfaces can introduce reflections, resulting in unwanted peaks in the transmitted signal. This problem is greater for smaller angles of incidence, where small multiple reflections may occur. Another limitation of the wedge tuning technique is that Snell's Law becomes invalid in cases where c.sub.p is less than c.sub.w. Consequently, angle wedge transducers cannot tune modes whose phase velocity falls below that of the longitudinal waves in the wedge. For example, the A.sub.0 mode in the low frequency range cannot be tuned using an angle wedge tuner, because c.sub.p, is less than 2,720 m/s as shown in FIG. 1. Yet another disadvantage in the angle wedge transducer comes from the fact that the wedge works as a delay block as a whole, requiring additional travel time that must be taken into account in the analysis of the received signal. Furthermore, the signal may be attenuated significantly before impinging the medium under test.
Another commonly used method for nondestructive evaluation involves the use of array transducers for single mode excitation of Lamb waves. One type of array transducer is a comb transducer. Another type of array transducer is an interdigital transducer. These devices are able to tune a desired mode by matching the transducer element spacing with a frequency of the excitation signal. Both of these array transducers are linear arrays having elements that are placed at a certain distance apart. A gated sinusoidal signal excites all the elements at the same time. By adjusting the distance between the elements, it is possible to generate guided waves of a wavelength equal to the distance between the elements.
Although array transducers can be more effective than the angle wedge transducer, there are disadvantages to using array transducers. The most critical problem is that the wave inherently propagates bidirectionally. This is because all of the transducer elements are simultaneously activated by the same signal, resulting in a symmetric excitation pattern. As a consequence, waves emanate from both sides of the transducer elements. Another disadvantage is that the transducer arrays cannot be effectively used as receivers because they are not able to accommodate the time delays introduced during reception.